I'm still learning my way around, currently combining DataForms and ListPages modules to try to link two sets of data together.
Got here while trying to find out if form_data{field} values can be passed by @URL

I would like to be able to use ListPages to help clients find records with missing data, so empty is helpful.
I can also see good reporting page applications for notempty - eg where a task completion date is available, listed by completion date, leaving out incomplete 'tasks' from the list.